I just got one. I got it for training though. I found on my full suspension, I was getting lazy; not getting out of the seat on climbs, counting on granny gear to get me up the hill, not standing over the rough stuff, etc. All of that changes on my rigid single speed. I'm forced to get out of the seat regularly, pulling back on the handle bars for climbs, getting in a good position for the climbs, etc. It is also a much better workout for the legs, arms and torso.
Not to mention the fun and challenge of it all. It reminded me of when I started and was learning stuff for the first time. It's always good to go back to your roots.
Oh, and for awhile there, I kept phantom shifting my first time out. I thought it was pretty funny.